I looked at our gnu.java.rmi.server.UnicastRef and - yes,
unfortunately it seems that there is system property in Classpath that
would force the object serialization even if the RMI client and server
are in the same machine.

If you plan serious testing work, we should probably add such property
as a necessary testing tool, but altering RMI implementation requires
to have a test suite first. I think, you can commit such tests anyway
because the object serialization is not the only thing in RMI that
deserves testing.

Unfortunately I do not plan for the moment serious rmi testing work. I
just wanted to report a bug within gnu classpath. Because the test
involves launching at least 2 vms, it's difficult to reveal the bug. The
property forcing serialization would be great in my case!